Getting Call-Response Behavior with Expect

[gnosti]

I would like to talk to an external program (midish) from perl using Expect (or Expect::Simple). I want to spawn this program once, keeping the child process loaded while I do other stuff. AFAIK Midish is well-behaved, using STDIN and STDOUT for I/O. However I'm having a hard time getting a reponse reliably after sending a command.

Update 1: I will investigate approaches other than Expect.

Update 2: Zentara's solution using IPC::Open3 is *way* simpler.

My first experiment was a simple command loop:

- get a prompt ("+ready")

- send a command

- get a reply

I thought I would try with Expect::Simple, however some buffering or command-ordering problems arise.

------ test-expect-simple ----

use Modern::Perl;
use Expect::Simple;

$| = 1;
my $obj = Expect::Simple->new({
     Cmd => [ midish => '-v'],
     Prompt => '+ready',
     DisconnectCmd => 'exit',
     Verbose => 0,
     Debug => 0,
     Timeout => 100
   });

sub prompt { print "midish> " }


while ( prompt(), my $cmd = <STDIN> ){
    
        exit if $cmd =~ 'quit';
        chomp $cmd;
        $cmd .= "\r";
        $obj->send( $cmd );
        my @lines = split "\n", $obj->before;
        splice(@lines, 0, 2); # throw away the first two lines
        say join "\n", @lines;
}

__END__

$perl test-expect-simple
midish> asdf

asdf: no such proc
midish> 1234

midish> ffff

3.5: statement or proc definition expected
midish> ^C

Comment: I'm not getting the full message after sending a command. Maybe it's a problem with Expect::Simple.

So here is the next try, a command loop using Expect.

------ test-expect-loop -----

use Expect;
use Modern::Perl;

my $exp = Expect->spawn("midish","-v")
    or die "Couldn't start program: $!\n";

# don't copy program output to STDOUT
$exp->log_stdout(0);

$exp->expect(1, '+ready', \&do_cmd);

sub do_cmd {

    map{say "midish: $_"} split "\n", $exp->before;
    print "enter command >> ";
    my $cmd = <STDIN>;
    $exp->send($cmd);
    exp_continue;
}

$exp->soft_close();

__END__

program output:


$ perl test-expect-loop
enter command >> adsfsd
midish: 
midish: adsfsd
midish: adsfsd: no such proc
enter command >> 1234
midish: 
midish: 1234
midish: 2.5: statement or proc definition expected
enter command >> ffff
midish: 
midish: ffff
midish: ffff: no such proc
enter command >> exit
$

Good, I'm getting output after every command. But my application is structured more like this:

call_to_expect($one_command);
execute_other_code();
call_to_expect($another_command);

------ test-expect-unrolled -----

use Expect;
use Modern::Perl;

my $exp = Expect->spawn("midish","-v")
    or die "Couldn't start program: $!\n";

# don't copy program output to STDOUT
$exp->log_stdout(0);

$exp->expect(1, '+ready', \&do_cmd);
print_output();

$exp->expect(1, '+ready', \&do_cmd);
print_output();

$exp->expect(1, '+ready', \&do_cmd);
print_output();

sub do_cmd {

    print "enter command >> ";
    my $cmd = <STDIN>;
    $exp->send($cmd);
}
sub print_output {
    map{say "midish: $_"} split "\n", $exp->before;
}

$exp->soft_close();
__END__

program output:

$ perl test-expect-unrolled
enter command >> asdf
enter command >> 1234
midish: 
midish: asdf
midish: asdf: no such proc
enter command >> ffff
midish: 
midish: 1234
midish: 2.5: statement or proc definition expected
<hang> 
^C
$

So, I still have a problem getting a one-to-one correspondence between the command string and the result string.

Any ideas how I could make one call to send a command and get a response, in a way that I can repeat indefinitely? Thanks for reading!